Power MOSFET devices are used in a variety of applications including audio/radio frequency circuits and high frequency inverters.
A typical power MOSFET is a vertical semiconductor device comprising an N.sup.+ substrate having a drain electrode on its lower surface. An N-drift layer is formed on the upper surface of the substrate. Diffused into the N-drift layer are one or more P-base regions. The P-base regions extend beneath a gate electrode situated atop an insulating layer which illustratively comprises silicon oxide on the upper surface of the substrate. One or more heavily doped N.sup.+ source regions are diffused into each base region. The N.sup.+ source regions are in contact with source electrodes.
The power MOSFET can operate in a forward conducting mode and a forward blocking mode. In the forward blocking mode, the gate and source electrodes are short-circuited. If a positive voltage is then applied to the drain electrode, the P-base/N-drift layer junctions are reverse biased and current flow is blocked. To carry current from drain to source in the power MOSFET (i.e., the forward conducting mode), it is necessary to form a conductive path extending between the N.sup.+ source regions and the N-drift region. This can be accomplished by applying a positive gate bias to the gate electrode. The resulting electric field attracts electrons to the surface of the P-base regions underneath the gate. The electric field is sufficient to create a surface electron concentration that overcomes the P-base doping in one or more narrow channels underneath the gate to provide a conductive path between the N.sup.+ source regions and the N-drift region. Now, the application of a positive drain voltage induces current flow between the drain and source electrodes through the N-drift region and the conducting channels formed in the P-base region. This current flow occurs solely by the transport of majority carriers. To switch the power MOSFET from the forward conducting to the forward blocking mode the gate bias is removed. When the gate bias is removed, electrons are no longer attracted to the region underneath the gate and the conductive path between drain and source is broken.
Some power switching circuits require provision for reverse current in active power switching devices. Some examples of these types of circuits are DC to AC inverters for adjustable speed motor drives, switching power supplies, and AC to DC choppers for motor speed control with regenerative braking. When using bipolar transistors, a diode connected in anti-parallel with the transistor has been used to conduct the reverse current. Illustratively, this diode is formed across the P-base/N-drift layer junction which forms an integral part of the power MOSFET discussed above. The anode current of the diode flows through the source electrodes which are in contact with the P-base regions. If the P-base/N-drift layer junction is forward biased, conduction takes place from source to drain through the injection of minority carriers into the drift layer.
The primary difficulty with using the integral diode of the power MOSFET is poor reverse recovery characteristics. Reverse recovery is the process whereby the diode is switched from its on state to its blocking state. To undergo this transition, the minority carrier charge stored in the drift layer during conduction must be removed. The stored charge removal occurs by means of two phenomena--the flow of a large reverse current, followed by recombination. The diodes integral with power MOSFET devices often exhibit very slow reverse recovery and very large reverse recovery currents. The reverse recovery current imposes power dissipation and thermal stress on various circuit components external to the power MOSFET.
Improvement of the reverse recovery characteristics of the integral diode in a power MOSFET was first accomplished by using election irradiation to introduce recombination centers into the drift layer. During reverse recovery, the lifetime reduction resulting from the irradiation can greatly reduce both the reverse recovery current and the recovery time. The reverse recovery characteristics can also be improved by gold or platinum doping of the drift layer to introduce recombination centers.
While the introduction of recombination centers into the drift region can improve the reverse recovery characteristics of the integral diode, the minority carriers stored in the drift region exhibit a sufficiently high lifetime that the resulting reverse recovery characteristics are not entirely satisfactory.
A further drawback to using an integral diode in the power MOSFET device is the forward voltage overshoot that occurs during the turn-on transient. This overshoot results from presence of the resistive drift layer. Under steady state reverse current conduction, the drift layer resistance is reduced by injected minority carriers. During high speed turn-on, however, the current rises at a faster rate than the diffusion of minority carriers injected from the junction. A high voltage drop develops across the drift region until the minority carriers can diffuse in and reduce the resistance.
A detailed discussion of conventional power MOSFET devices including use of an integral P-N junction to conduct reverse current and the drawbacks associated therewith may be found in B. J. Baliga, "Modern Power Devices", John Wiley & Sons, 1987, pp. 263-342.
Accordingly, it is an object of the present invention to provide a power MOSFET device with an integral diode for conducting reverse current, which diode exhibits very good reverse recovery and forward voltage overshoot characteristics.
As indicated above, it is sometimes necessary to accommodate reverse current in connection with use of a bipolar power transistor. This need may typically arise in motor control applications. Therefore, it is a further object of the invention to provide a bipolar transistor with an integral diode for conducting reverse current, which diode exhibits very good reverse recovery and forward voltage overshoot characteristics.